Fuel injection systems deliver fuel to the combustion chamber of a gas turbine engine, where the fuel is mixed with air before combustion. One form of fuel injection system well-known in the art utilises fuel spray nozzles. These atomise the fuel to ensure its rapid evaporation and burning when mixed with air.
An airblast atomiser nozzle is a type of fuel spray nozzle in which fuel delivered to the combustion chamber by a fuel injector is aerated by air swirlers to ensure rapid mixing of fuel and air, and to create a finely atomised fuel spray. The swirlers impart a swirling motion to the air passing therethrough, so as to create a high level of shear and hence acceleration of the low velocity fuel film.
Typically, an airblast atomiser nozzle will have a number of coaxial air swirler passages. An annular fuel passage between a pair of swirler passages feeds fuel onto a prefilming lip, whereby a sheet of fuel develops on the lip. The sheet breaks down into ligaments which are then broken up into droplets within the shear layers of the surrounding highly swirling air to form the fuel spray stream that enters the combustor.
FIG. 1 shows schematically a longitudinal cross section through a conventional fuel spray nozzle 132 which injects a pilot flow of air and fuel and a mains flow of air and fuel into a combustor 130. The nozzle comprises a pilot airblast fuel injector having an annular fuel passage 134 which allows the fuel to flow as a film on an annular prefilmer surface. A pilot inner swirler 136 located on the centerline 135 of the nozzle and a pilot outer swirler 138, are used to swirl air past the film, causing the liquid fuel to be atomized into small droplets.
The fuel spray nozzle 312 further includes a mains airblast fuel injector which is coaxially located about the pilot airblast fuel injector. The mains airblast fuel injector has inner 142 and outer 144 main swirlers which are located coaxially inward and outward of a mains fuel passage 140.
All four swirlers 136, 138, 142 and 144 are fed from a common air supply system, and the relative volumes of air which flow through each of the swirlers are dependent upon the sizing and geometry of the swirlers and their associated air passages. Each swirler comprises a circumferential row of vanes. The two swirlers of each of the pilot and the mains fuel injectors may be either co-swirl or counter-swirl.
In the conventional fuel spray nozzle 132, the vanes of a given swirler extend generally radially, as depicted in FIG. 2, which shows schematically the trailing edges 146 of a row of vanes as viewed looking upstream along the respective air passage. In addition, to reduce slippage of air leaving the vane trailing edge, the vanes may be twisted so that the chordal lines of successive aerofoil sections are at increasing stagger angle with increasing radial height. An aim is to achieve a direction of flow leaving the vanes that is at a tangent to the pitch circle at all vane radial heights, as shown by the dashed arrowed lines in FIG. 2.
FIG. 3 shows an enlarged view of the mains inner swirler 142, its corresponding air passage 148, and an outlet port 150 of the mains fuel passage 140 of the fuel spray nozzle 132 of FIG. 1. The swirler is located in a cylindrical section of the air passage. In a following section, the air passage diverges (i.e. turns radially outwards). In the longitudinal cross sectional view of FIG. 3, the transition between the cylindrical and divergent sections appears as a bend 152 in the passage. The outlet port takes the form of an annular slot in the outer side wall of the air passage downstream of the bend. Fuel fed through the outlet port develops into a film on a frustoconical prefilmer surface 154 of the outer side wall. The swirling air flow (indicated by dotted arrowed lines) exiting the swirler travels along the air passage. In the divergent section, the flow area of the air passage may decrease, accelerating the air flow and helping it to atomize the liquid fuel film into small droplets.
The present invention is at least partly based on a recognition that, as a result of the bend 152, a thick boundary layer 156 can develop in the vicinity of the outlet port 150 and over the prefilmer surface 154. This boundary layer can reduce the effectiveness of the air flow in atomizing the fuel film. A related problem is that the bend itself can produce losses in the air flow, as it is forced by the bend to change direction.